It is known that stabilized zirconium oxide powders can be prepared in a hydrothermal manner (see U.S. Pat. No. 4,619,817).
In that process, an aqueous zirconium salt solution is mixed e.g. with the complexing agent ethylene diamine tetraacetic acid, as well as with stabilizing elements calcium and/or magnesium and/or yttrium. Subsequently the mixture is titrated with sodium liquor solution to a pH greater than 11.
The mixture, which has a concentration of sodium hydroxide solution of more than 4 moles per liter, is treated in an autoclave hydrothermally at temperatures of about 190.degree. C. The product is tetragonal or cubic zirconium oxide.
According to examples 24 and 25 of U.S. Pat. No. 4,619,817, no complexing agent is added; however, the concentration of sodium hydroxide solution is very low after titration with NaOH with 0.29 mole/l.
The method described in this patent has the following disadvantages:
The addition of a complexing agent brings about a lowering of the pH. This results in the formation of powders with specific surface areas greater than 30 square meters per gram. The resulting very small primary crystallite size leads, during drying of these powders, to the formation of agglomerates. This considerably aggravates or prevents processing in shaping methods which contain a dry or semidry pressing step. The pressing process results in the formation of tensions in the green body and also in the sintered body.
The complexing agent and the decomposition products of the complexing agent cause corrosion, and thus lead to a considerable limitation on the kinds of metallic materials which can be used, and thus to increased costs. The use of a polytetrafluoroethene jacket in the autoclave, as described in the aforesaid U.S. patent, limits the temperature range which can be employed.
Non-homogeneous metal hydroxide precipitates are generated by the titration of the acidic Zr/Y/Mg salt solution with NaOH on account of the fractional precipitation.
The low NaOH concentrations selected in the examples of U.S. Pat. No. 4,619,817, concentrations of &lt;0.3 mole/liter, and the low pH values associated therewith, lead to the disadvantageous, high specific surface areas described in this patent.